Continued from page 4
- Case Study 1
- Case Study 2
- Appendix A - Fertilizer Recipe
- Comments on this Paper
- Later comments on this Paper
Plants cannot grow without phosphate. However, in order to keep a planted aquarium relatively algae free, free phosphate in the water column must be minimized. The following recommendations will help achieve this goal:
(a) A slight excess of light, CO2, K, N, micronutrients, and trace elements should be maintained to allow the plants to utilize all of the available phosphate. The authors recommend the following:
- 20-60 lumens/L illumination (about 2-4W fluorescent light per gallon), 12h/day
- 10-15ppm CO2
- 3-5ppm NO3
- 0.1ppm Fe
- 6.5-7.0 pH
Since inexpensive tests are not available for trace elements, micronutrients, or K, these items are dosed as some percentage of the measurable nutrients. The authors have had considerable success with mixtures that duplicate the relative concentrations present in Tropica Master Grow fertilizer . For those readers wishing to "roll their own", a balanced fertilizer recipe is given in the Appendix. Various commercial aquatic plant fertilizers are also available, but it may be necessary to purchase several products to ensure complete nutrient and trace element coverage. Daily dosing is highly recommended because it may prevent temporary nutrient depletion, which could make phosphate available on an intermittent basis and prevent the algae from starving.
As a general approach to optimizing plant growth and reducing algae, the following procedure is suggested:
- Set the light and CO2 levels.
- Add an iron-containing trace element mix (preferably one that already has Mg) to the tank every day, adjusting the quantity on a regular basis to achieve the target iron level. For mixes without Mg, add Epsom salts as well in the ratio of about 1.5-5.0ppm Mg to 1ppm Fe.
- A week or so after reaching the target Fe level, check the nitrate level. If nitrates are below about 2ppm, proceed to the next step. Otherwise, add enough K2SO4 to the tank every day to drop the nitrate level to as close to zero as possible and keep it there (if the nitrates don't drop, then something other than K is limiting plant growth and some detective work will be required to find it). Incidentally, measuring the nitrate level is helpful for general tweaking; if adding nutrient X causes the nitrate level to drop, then the tank is probably deficient in X.
- Add enough KNO3 to the tank every day to get a 3-5ppm nitrate reading (one of the authors [Conlin] obtains satisfactory results with 10ppm).
Once the relative amounts of trace elements, K2SO4, and KNO3 have been determined, it becomes a simple matter (if desired) to concoct a liquid fertilizer that can be poured into the tank each day. Using a mix of dry powders is not recommended as powders tend to separate.
The procedure just described ensures that there will always be a slight excess of nitrogen in the tank. Some terrestrial plants will not flower if nitrogen is abundant, and this may be the case for some aquatic plants too. It would be an interesting experiment to withhold fertilization for several weeks after a lengthy period (say 6 months to a year) of good plant growth to attempt to induce flowering.
There is a possibility that some of the trace elements will accumulate over time to levels toxic to plants if regular water changes are not done. 25% water changes every second week should prevent this from happening.
(b) Grow fast-growing plant species that can efficiently extract nutrients directly from the water column. These plants will rapidly strip phosphate from the water, making it unavailable to algae. Floating plants (Lemna minor, Limnobium laevigatum) and stem plants that grow roots at internodes (Hygrophila sp.) are suggested for this purpose.
(c) Enriched substrates are probably the best means of supplying phosphates to plants provided steps are taken to minimize the leakage of phosphate into the water column. Substrate fertilizers such as Pond Tabs should be buried deep in the substrate where their nutrients are preferentially available to plant roots. Substrate circulation should be minimized to prevent phosphate from leaching too rapidly into the water column. Avoid gravel cleaning and other substrate disturbances if at all possible. Eliminating substrate circulation completely would not be desirable (even if it were possible) because supplementary fertilizers are usually added to the water and must be transported to the roots somehow.
(d) There will always be some residual algae in a planted tank because it is impossible to keep the water completely phosphate free. The amount of residual algae will be very small, but a good selection of algae-eating fish (Otocinclus sp., Farlowella sp., Ancistrus sp., Crossocheilus siamensis) and invertebrates (Caridina japonica shrimp and some snails) is desirable anyway for controlling the algae outbreaks that occur when the tank is first set up, the substrate is disturbed, or the nutrients are incorrectly dosed.
(f) Do not use phosphate buffers to control pH. Use of these buffers may produce phosphate concentrations as high as 100ppm, almost certainly resulting in very impressive algae blooms.
(g) Algicides such as simazine and copper are not recommended because they damage plants and may be unhealthy for fish as well .
(h) Miscellaneous considerations:
Tap water is not recommended as a source of trace elements because it may be deficient in one or more elements, and rapid plant growth is likely to deplete the elements far more quickly than they can be replaced.
Certain water treatment products (Aquasafe, NovAqua) should be avoided as they bind metals (including iron), making them unavailable to plants. They may also contain phosphate buffers. Simple dechlorinators or products such as Amquel are a better choice for treating tap water during water changes.
Carbon filtration may remove necessary trace elements from the water. With regular water changes and good plant growth, carbon filtration is not necessary and should be omitted.
(i) As a general principle, avoid adding fertilizers, water treatments, or any other products to one's aquarium unless the products completely disclose the concentration of each ingredient present. Otherwise, there is no way to knowing what effect (if any!) these products will have on the aquarium's inhabitants.
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